The invention relates generally to systems which use and generate images during medical and surgical procedures, which images assist in executing the procedures and indicate the relative position of various body parts and instruments. In particular, the invention relates to a system for generating images during medical and surgical procedures based on a scan taken prior to or during the procedure and based on the present position of the body parts and instruments during the procedure.
Image guided medical and surgical procedures comprise a technology by which scans, obtained either pre-procedurally or intra-procedurally (i.e., prior to or during a medical or surgical procedure), are used to generate images to guide a doctor during the procedure. The recent increase in interest in this field is a direct result of the recent advances in scanning technology, especially in devices using computers to generate three dimensional images of parts of the body, such as computed tomography (CT) or magnetic resonance imaging (MRI).
The majority of the advances in diagrammatic imaging involve devices which tend to be large, encircle the body part being imaged, and are expensive. Although the scans produced by these devices depict the body part under investigation with high resolution and good spatial fidelity, their cost usually precludes the dedication of a unit to be used during the performance of procedures. Therefore, image guided surgery is usually performed using images taken preoperatively.
The reliance upon preoperative images has focused image guidance largely to the cranium. The skull, by encasing the brain, serves as a rigid body which largely inhibits changes in anatomy between imaging and surgery. The skull also provides a relatively easy point of reference to which fiducials or a reference system may be attached so that registration of pre-procedural images to the procedural work space can be done simply at the beginning, during, or throughout the procedure. Registration is defined as the process of relating pre-procedural or intra-procedural scan of the anatomy undergoing surgery to the surgical or medical position of the corresponding anatomy. For example, see Ser. No. 07/909,097, now U.S. Pat. No. 5,383,454 the entire disclosure of which is incorporated herein by reference.
This situation of rigid fixation and absence of anatomical movement between imaging and surgery is unique to the skull and intracranial contents and permits a simple one-to-one registration process as shown in FIG. 1. The position during a medical procedure or surgery is in registration with the pre-procedural image data set because of the absence of anatomical movement from the time of the scan until the time of the procedure; in effect, the skull and it's intracranial contents comprise a "rigid body," that is, an object which does not deform internally. In almost every other part of the body there is ample opportunity for movement within the anatomy which degrades the fidelity by which the pre-procedural scans depict the intra-procedural anatomy. Therefore, additional innovations are needed to bring image guidance to the rest of the body beyond the cranium.
The accuracy of image guided surgery relies upon the ability to generate images during medical and surgical procedures based on scans taken prior to or during the procedure and based on the present position and shape of the body parts during the procedure. Two types of body parts are addressed herein: 1) structures within the body that do not change shape, do not compress, nor deform between the process of imaging and the medical procedure, which are termed "rigid bodies," and are exemplified by the bones of the skeleton; and 2) structures within the body that can change shape and deform between the process of imaging and the medical procedure structures are termed "semi-rigid bodies," and are exemplified by the liver or prostate. Both types of body parts are likely targets for medical or surgical procedures either for repair, fusion, resection, biopsy, or radiation treatment. Therefore, a technique is needed whereby registration can be performed between the body parts as depicted pre-procedurally on scans and the position and shape of these same body parts as detected intra-procedurally. This technique must take into account that movement can occur between portions of the body which are not rigidly joined, such as bones connected by a joint, or fragments of a broken bone, and that shape deformation can occur for semi-rigid bodies, such as the liver or prostate. In particular, the technique must be able to modify the scanned image dataset such that the modified image dataset which is used for localization and display, corresponds to position and/or shape of the body part(s) of interest during a medical or surgical procedure. A key to achieving this correspondence is the ability to precisely detect and track the position and/or shape of the body part(s) of interest during the medical or surgical procedure, as well as to track instruments, ------- or radiation used during the said procedure.